[gentoo-portage-dev] allow people to disambiguate built_with_use in upgrade situations

[Mike Frysinger]

Diego is proposing a new flag to built_with_use to handle the corner case 
where you query a package that does not have the requested flag in IUSE

built_with_use [--missing <action>] ...

so in the case of version transitions, you can specify the <action> as either 
true, false, or die (only current option)

seems like a cleaner solution than forcing what can be a rats nest of 
has_version checks
-mike